GPS and Gas Tubes


Global positioning systems are an important part of any RF network. GPS provides timing information that is critical in keeping RF networks synchronized. It is important that GPS receivers be protected from lightning strikes with RF lightning arrestors. 

RF lighting arrestors come in many forms and use different technologies to provide protection. A circuit protection component, such as a gas tube or diode, are often used in a RF lightning arrestor. Figure 1 below shows what the general strategy is behind using a protection device in a circuit: when the protection device turns on, its impedance is much lower than the equipment. With a low impedance, most of the energy is diverted to ground.

Figure 1: Typical strategy of a protection device in a circuit: to have a lower impedance than the load and to shunt away a majority of the lightning energy. 

A gas tube is a popular protection device that is used in an RF lightning arrestor. A gas tube activates when the gas inside it ionizes and conducts. The drawback with a gas tube is that it typically requires several hundred volts to get it to activate, which can take several microseconds. As a result the equipment being protected by a gas tube is exposed to several hundred volts and several microseconds of lightning current.

Other RF lightning arrestors will incorporate the gas tube as well as other circuit protection devices. For example, the circuit in Figure 2 outlines a hybrid circuit. This circuit does use a gas tube, but also uses a diode and a metal oxide varistor (MOV). The advantages of this circuit is it incorporates the reaction time of a diode and MOV, which address the disadvantages of a gas tube. During a lightning strike the diode turns on quickly (in nanoseconds) which then causes the MOV to turn on (sub-microseconds), which then causes the gas tube to turn on (microseconds). The circuit reacts fast enough that it provides the lowest and energy let through of any diode, MOV or gas tube based RF surge arrestor in the market. 

When looking for the highest levels of protection, using a hybrid protector is the best option available for GPS receivers. Gas tube based RF arrestors, though inexpensive, over the least amount of protection against lightning. 

Figure 1: A hybrid circuit incorporates a diode and MOV, resulting in a protection circuit that reacts in nanoseconds.